In recent years, demand for high-speed signal processing, high function, miniaturization of a product, and reducing the thickness of a product shape in an electronic device circuit in association with digital electronic equipment, has been being accelerated. Data processing speeds of various types of semiconductor devices mounted on a printed circuit board (PCB) become high, and thus the component mounting density of active components and passive components mounted on the printed circuit board (PCB) is also going high.
Such components accompany a rapid change of the voltage and current by a high-speed signal, to thus generate inductive noise as a source of generating high frequency noise. These components such as active components and passive components are located very close together on a printed circuit having printed wires, to thereby cause cross-talk problems caused by electromagnetic coupling between internal components or between the components and the wires in small digital electronic devices, or radiate electromagnetic waves out of the small digital electronic devices to thereby cause EMI (ElectroMagnetic Interference: EMI) or electromagnetic wave interference problems which affect the other devices.
In addition, electromagnetic waves generated from electronic devices which are long-term exposed to the human body, may affect the human body, to thus cause diseases such as glaucoma, and reduction of fertility.
The electromagnetic shielding which is proposed as measures against EMI problems means that the electromagnetic noise generated from the inside of the electronic device is not radiated to the outside of the case, and also electromagnetic noise entering from the outside is absorbed or blocked.
In the case of digital electronic devices that have a spatial room in the inside of the digital electronic devices, EMI measures are taken as follows. Filters are connected to circuits which generate inductive noise, to thus remove the inductive noise or distances between the circuits get far away from each other, to thus prevent cross-talk problems caused by electromagnetic coupling between the circuits. In addition, the digital electronic devices are shielded with electromagnetic wave shielding materials to then be grounded.
However, since electronic components are mounted at a high density on printed circuit board (PCB) in the case of small digital electronic devices, the above-described measures against noise are not suitable as urgent measures against noise for products of a short product life. This is because mounting spaces are insufficient for mounting filters in the small digital electronic devices, and miniaturization and thinning should be taken in a design stage.
For this reason, recently, soft-magnetic composite magnetic body sheets of about 0.2 mm or more relatively thick, are used in input/output (I/O) ends, in order to inhibit inductive electromagnetic noise generated by active components that are major noise sources on the circuit boards of the small digital electronic devices.
The permeability of materials for the composite magnetic body sheets is composed of a real part permeability component and an imaginary part permeability component. A suppression efficiency of the noise becomes large as the permeability of the imaginary part in the electromagnetic noise frequency band gets large, and as the composite magnetic body gets thick.
Meanwhile, since the size of the digital electronic devices becomes more compact on trend, products having the thin composite magnetic body and the excellent suppression effect of the electromagnetic wave are required. In addition, in accordance with the above-mentioned miniaturization of the electronic devices, thinning of the composite magnetic body sheets for measures against noise used for a quasi-microwave band is required.
For the composite magnetic body sheets, the imaginary part permeability should become large in order to achieve thinning by using a noise reduction effect due to a magnetic loss. The current magnetic material has a problem that it does not meet the thinning and the conductive noise suppression effect, in a lower frequency band or a higher frequency band than about 10˜100 MHz.
Korean Patent No. 10-0755775 disclosed a composite structure noise suppressing film and a method of manufacturing the same, in which in order to increase a reduction effect of electromagnetic waves even with a thin type having a thickness of about 25 to 100 μm, a plate-like resistance body powder and a plate-like soft-magnetic material powder are oriented in an insulating polymer matrix, to thus exhibit shape anisotropy, and to thereby provide an electromagnetic wave reduction effect by a resistance loss and a magnetic loss, and improve the electromagnetic wave reduction effect remarkably in frequencies of 1 GHz or above.
The Korean Patent No. 10-0755775 has a limit to reduce the thickness since a plate-like carbon powder that is a resistance body powder and a plate-like Sendust® powder that is a soft-magnetic material powder are oriented in an insulating polymer matrix, and has low imaginary part permeability because of the relatively low permeability when compared with those of different types of magnetic sheets, to thereby increase a sheet price because of the use of an expensive soft magnetic powder of a low absorbing rate.
In addition, in the case of a polymer magnetic sheet containing the soft-magnetic powder, and in the case of increasing the thickness of the sheet to improve the low magnetic permeability, the material cost is further increased with an increase in the thickness, and it is difficult to cope with the thinning tendency of mobile terminals. Furthermore, the noise suppressing film is commercially available as a thick film of 0.2 mm or above and thus has a limit in use.
Meanwhile, Korean Patent Laid-open Publication No. 10-2011-92833 proposed an electromagnetic wave absorbing sheet containing a Fe-based nanocrystalline soft-magnetic powder and a carbon-based conductive material powder. The Fe-based nanocrystalline soft-magnetic powder is formed of a Fe—Si—B—Nb—Cu-based alloy as an amorphous alloy. The Fe—Si—B—Nb—Cu-based alloy is preliminarily heat treated at a temperature of 350° C. to 500° C. for 45-90 minutes, to thus obtain alloy powders, the alloy powders are primarily and secondarily crushed, and then the crushed powders are meshed to be 270 mesh or less in particle size, to thereby obtain Fe-based nanocrystalline soft-magnetic powders having nano-sized crystal grains.
However, the electromagnetic wave absorbing sheet is a kind of a polymer sheet that is made to have a thickness of 0.5 mm by mixing a Fe-based nanocrystalline soft-magnetic powder having nano-sized crystal grains, with a binder. Accordingly, the electromagnetic wave absorbing sheet gets thick and also has the low magnetic permeability of the sheet due to the mixture of the binder.
Korean Patent Laid-open Publication No. 10-2005-37015 discloses a metal and polymer composite having a low frequency magnetic field shielding function, which includes at least one selected from Permalloy®, Sendust®, and a rapidly solidified alloy that are metal alloys having a low frequency high magnetic shielding function as a main component in one of a powder-like, flake-like, or fiber-like form, and a soft polymer material as a matrix in which a metal alloy is dispersed, and an additive which is used to composite the metal alloy and the soft polymer material. However, the sheet proposed in Korean Patent Laid-open Publication No. 10-2005-37015 is a kind of the polymer sheet, and has a low permeability problem.
Korean Patent Laid-open Publication No. 10-2003-86122 discloses a method of manufacturing an electromagnetic wave shielding material by using a metal foil ribbon of high magnetic permeability, the method including: preparing a metal foil ribbon in the range of 1 μm to 900 μm thick, and in the range of 1 mm to 90 mm wide, with a metal or alloy having a specific permeability of 1000 or more and selected from Ni—Fe—Mo, Fe—Si, and mu-metal by performing a quenching solidification method; annealing the metal foil ribbon in a temperature range of 700° C. to 1300° C. and under a hydrogen or vacuum atmosphere; and forming an adhesive layer on at least one surface of the metal foil ribbon.
Further, the electromagnetic wave shielding material manufacturing method further comprises forming a thin film layer of Cu, Ni, Ag, Al, Au, and Sn or a combination of these metals on at least one surface of the metal foil ribbon by electroplating or vacuum deposition.
However, the electromagnetic wave shielding material manufactured according to the Korean Patent Laid-open Publication No. 10-2003-86122 shield electromagnetic waves in a way of reflecting most of the electromagnetic waves by the metal foil ribbon and the metal film coated or vacuum deposited on the metal foil ribbon, but does not shield electromagnetic waves in a manner that absorbs the electromagnetic waves. As a result, when the electromagnetic shielding material is used in the inside of the electronic device, adjacent other circuit elements are affected by the electromagnetic wave reflected from the electromagnetic shielding materials.
In addition, Korean Patent Laid-open Publication No. 10-2009-123776 discloses a method of producing a conductive foam sheet which can be used as a material of an electromagnetic wave shielding gasket.
The conductive foam sheet for the electromagnetic wave shielding gasket includes: a foam sheet whose polarity is imparted by a coupling agent; a nickel plating layer formed on the foam sheet by 1000 Å to 3000 Å thick with an electroless plating method; and a copper plating layer formed on the nickel plating layer with a thickness of 0.5˜3.0 μm under conditions of a current density of 1.3 to 5.0 A/dmin2 by using an electrolytic plating method in which a surface resistance value is 0.02 to 0.08 Ω/square.
However, the above Korean Patent Laid-open Publication No. 10-2009-123776 shields electromagnetic waves in a way of reflecting most of the electromagnetic waves by the nickel and copper plating layers, but does not shield electromagnetic waves in a manner that absorbs the electromagnetic waves. As a result, when the conductive foam sheet is used in the inside of the electronic device, adjacent other circuit elements are affected by the electromagnetic wave reflected from the conductive foam sheet.
Korean Patent Laid-open Publication No. 10-2009-78620 discloses an electromagnetic wave shielding sheet formed of a substrate sheet formed of a synthetic resin film, and a metal deposition layer which is sputtered on a surface of the substrate sheet, in which the metal deposition layer includes at least one selected from the group consisting of copper (Cu), nickel (Ni), zinc (Zn), aluminum (Al) and manganese (Mn).
However, in the same manner as that of the Korean Patent Laid-open Publication No. 10-2009-123776, the electromagnetic wave shielding sheet disclosed in the Korean Patent Laid-open Publication No. 10-2009-78620 may cause a problem of affecting adjacent other circuit elements due to the electromagnetic wave reflected from the metal deposition layer.
Since the substrate sheet is formed of the synthetic resin film, the shielding sheet is thick and thus the overall thickness of the portable electronic device increases. As a result, in general, the mobile terminals seeking lightweight, thinning, shortening, and compactness may not use the shielding sheet when the thickness of the shielding sheet exceeds 0.5 mm.
Further, when depositing a conductive metal layer on a surface of the synthetic resin film, electromagnetic wave shielding performance may be reduced since the peeling of the conductive metal layer may occur.